1. Field of the Invention
The present invention relates to a packet signal selecting apparatus for selecting one of a plurality of packet signals respectively transmitted through a plurality of communication paths in a mobile communication system, and a mobile switching center including the apparatus and realizing macro-diversity.
2. Description of the Prior Art
A conventional mobile communication system for vehicle telephone sets and portable telephone sets uses a small zone scheme of covering the entire service area of a mobile communication system with radio zones at a plurality of locations. In this case, a radio zone is a range in which each radio base station communicates between mobile units mounted in vehicles through radio channels. One radio base station is installed in one radio zone, and a plurality of mobile units freely move between the respective zones. Each radio base station can use a plurality of radio channels and determines a radio channel to be used on the basis of a radio wave propagation environment between mobile units between which communication is to be performed. In addition, if the radio wave propagation environment for a radio channel in use deteriorates during communication, another radio channel is used. This operation is called a "channel switching during voice communication" (handover). A mobile switching center is an apparatus for controlling these radio base stations and performing channel switching with respect to a fixed telephone network.
As shown in FIG. 1, in a digital mobile communication system (Nakajima et al., "Digital Mobile Communication System Plan" in B-317 of Proceedings of the 1991 IEICE Spring Conference, and Ishino et al., "Mobile Communication Switching System for Voice/Nonvoice Services", IEEE GLOBECOM '91, pp. 1485-1489), a mobile switching center 1 is constituted by a mobile switching unit 3 accommodating a nonvoice/ISDN adaptor 2, voice processing units 4 and 5, and a triple connection trunk 6. A radio base station 10 is constituted by modulation/demodulation units 11 and 12, a control unit 13, and common amplifiers 14 and 15. The voice processing units 4 and 5 are signal compressors each constituted by a CODEC and the like.
Channels between the mobile switching center 1 and the radio base station 10 are designed as follows. As for a transmission path 18 for control signals, in order to improve the reliability, 64-kbps channels are duplexed. As for a transmission path 19 for voice signals, 11.2-kbps voice signals output from the voice processing units 4 and 5 on a 64-kpbs transmission path are multiplexed by three-channel time-division multiplexing in a synchronous terminal station 16 on the mobile switching center 1 side, and are shunted in a synchronous terminal station 17 on the radio base station 10 side, thereby achieving a reduction in channel cost. In addition, a triple connection trunk 6 is used to connect an old radio base station, a new base station, and a public network to each other before radio channel switching is performed at the time of handover, thereby preventing a hit.
In the conventional mobile communication system shown in FIG. 1, since the degree of multiplexing in a transmission path needs to be changed in accordance with the bit rates of the voice processing units 4 and 5, it is difficult to use different bit rates. In addition, the triple connection trunk 6 specially designed for triple connection is required at the time of handover. In order to compensate for these drawbacks, a scheme of using packet signals for transmission between a mobile switching center and a radio base station has been proposed (Japanese Patent Application Nos. 4-181405 and 4-183269). When packet transmission is performed between a mobile switching center and a radio base station, various bit rates can be used by changing the number of packet signals transmitted per unit time. Therefore, multiple connection can be realized by only copying a packet signal and changing the destination address of each packet signal, and no special device such as the triple connection trunk 6 is required.
Furthermore, attempts have been made to use a hand-off scheme of preventing a hit at the time of handover and compensating for a deterioration in signal quality by using macro-diversity in which radio signals from a mobile unit are received by a plurality of radio base stations to improve the signal quality (refer to "International Publication No. WO92/0063 published Jan. 9, 1992"). In order to realize macro-diversity, a mobile switching center 50 is designed as follows. As shown in FIG. 2, signals from a plurality of radio base stations are subjected to switching in a digital switching unit 52. Thereafter, these signals are output, as one signal, from a diversity synthesizer 55 to a voice processing unit 54. The signal is then connected to a public network through a digital switching unit 51 or is returned by the digital switching unit 51 to be connected to a mobile unit within a mobile communication network through the voice processing unit 54 and the digital switching unit 52.
In the conventional mobile switching center shown in FIG. 2, however, since the diversity synthesizer 55 is directly connected to the voice processing unit 54, diversity synthesizers 55 equal to voice processing units 54 in number are required. In addition, in communication between mobile units, a voice signal passes through the voice processing unit 54 twice, resulting in a deterioration in signal quality. Furthermore, since the diversity synthesizer 55 receives signals from the digital switching unit 52 through a plurality of input terminals, complicated input destination management is required in the digital switching unit 52 for signals from radio base stations.